Method of assembling and cooling a package structure with accessible chip

ABSTRACT

A package for an integrated-circuit includes a package body having a die-cavity formed therein. A die-attach pad is formed in the package body adjacent the die-cavity. An opening is formed in the central portion of the die-attach pad for exposing one side of the integrated-circuit die so that an external cooling media can directly contact the exposed side of the integrated-circuit die. The die-attach pad can be formed as a die-mounting ring adjacent the die-attach cavity. The peripheral edge of the integrated-circuit die is fixed to a mounting surface on the die-mounting ring portion to accommodate direct cooling of the exposed side of the integrated-circuit die. The mounting surface of the die-mounting ring extends beyond the peripheral edge of the integrated-circuit die to accommodate a range of sizes of the integrated-circuit die. The exposed surface of the integrated circuit die is cooled, for example, with a cooling fluid, a heatsink, or a thermo-electric refrigeration unit in contact with the exposed side of the die. The exposed side of the die is coated with a film to provide a seal for the exposed side of the integrated-circuit die.

This application is a continuation of application Ser. No. 08/096,333,filed Jul 23, 1993, now abandoned, which is a divisional of applicationSer. No. 07/876,249 filed Apr. 30, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to packaging for integrated-circuit chips and,more particularly, to packaging techniques which provide improvedcooling for an integrated-circuit chip.

2. Prior Art

Efficient removal of heat frown a packaged integrated-circuit chip isvery important because the temperature of the integrated-circuit chipmust be kept below certain designated temperatures to avoid degradedperformance or to prevent damage to the chip. Very often, a siliconintegrated-circuit chip is enclosed in a package, where the packageitself has a significant thermal resistance. Heat energy travels inseries from its source in the integrated-circuit through the thermalresistance of the package to the ambient. For a given amount of heatpower being generated by the integrated-circuit chip, higher values ofthermal resistance produce higher temperatures in the integrated-circuitchip. The overall thermal resistance for the series combination of thechip and the package is always greater than the thermal resistance ofthe package alone so that the thermal resistance of the package oftenlimits the thermal performance of the integrated-circuit chip.

Two commonly used packages for integrated-circuit chips are apin-grid-array (PGA) package and a plastic quad flat pack (PQFP) packageconfigurations. The PGA package is square and has a number of downwardlyextending, uniformly-spaced terminal pins arranged on a grid. A PQFPpackage has bent, flat leads extending from all four sides of thepackage. Both types of package structures enclose a silicon chip andhave thermal resistance, preventing direct removal of heat from theintegrated-circuit chip. No matter how well the ambient removes heatfrom the package, the package still remains as an insulator. It is notpossible to achieve a thermal resistance lower than the thermalresistance of the package.

FIG. 1 shows a standard pin-grid-array PGA package assembly 10. Theassembly includes a package body 12 formed of a ceramic or moldedplastic material. A recessed cavity 14 is formed in a front side of thepackage body 12 for receiving an integrated circuit die, or chip, 16.The integrated-circuit die 16 is fixed with a layer 18 of die-attachmaterial to the interior surface of a rear wall 20 of the package body12, where the rear wall 20 defines the interior boundary of the cavity14. Wire-bonding pads on the surface of the integrated-circuit die 16are connected by bonding wires (typically shown as 22, 24) tocorresponding bonding-pad portions of internal conductors (typicallyshown as 28) contained within the package body 12. The internalconductors 28 are connected with conventional feed-through solderconnection means to respective connection pins (typically shown as 30,32). A problem with this conventional PGA package configuration is thatheat from the integrated-circuit die 16 must travel through thethickness of the wall 20 and other parts of the body of the package toescape to the ambient. Consequently, the thermal performance of thepackage is limited by the thermal coefficient of the wall material andthe thickness of the wall 20.

A number of prior solutions are available for lowering the thermalresistance of a standard PGA package configuration for anintegrated-circuit chip. One solution is to use a package material whichhas a better thermal conductivity. For example, a ceramic material canbe used instead of a plastic material for the packaging material.

FIG. 2 shows another technique for improving the thermal performance ofa pin-grid-array PGA package assembly 50. The pin-grid-array PGA packageassembly 50 is similar to the package assembly of FIG. 1 and the samereference numerals are used for like elements. The pin-grid-array PGApackage assembly 50 uses an additional component within a package body52 for the package assembly 50. That component is an internal heatspreader 54, which is a plate, or slug, formed of copper orcopper/tungsten material. The integrated-circuit chip 16 is bonded tothe heat spreader 54 with a layer 56 of suitable die-attach material, asindicated in the Figure.

Use of the heat spreader 54 improves the heat transfer characteristicsof the package 50, but the heat spreader 54 is still in the seriesthermal path from the ambient to the integrated-circuit chip 16. Asignificant problem with using a heat spreader 54 is that cracks developin the body of the package 52 near the junction of the heat spreader 54and the package 52. As the metal of the heat spreader 54 expands andcontracts with changes in temperature, the stronger, more rigid metalheat spreader exerts stress on the package body 52 and also on theintegrated-circuit die 16, causing the package material to crack. Toreduce the amount of stress on the package body, the heat spreader 54 iskept small, that is, approximately the same size as the area of theintegrated-circuit die 16.

FIG. 3 shows another type of conventional package assembly forintegrated-circuits. A standard plastic quad flat-pack package PQFP 100includes a package body 102 formed of molded plastic material. Themolded-plastic body 102 is molded around an integrated-circuit die 104,which is fixed to a die-attach pad 106 portion of a standard leadframe,using a suitable layer of die-attach material 108, as indicated in theFigure. Wire-bonding pads on the surface of the integrated-circuit die104 are connected by bonding wires (typically shown as 110, 112) tocorresponding bonding finger portions of leads (typically shown as 114,116), which extend out of the package body 102, as indicated in theFigure.

Similar to the case of the standard PGA package, a problem with theconventional PQFP package configuration is that heat from theintegrated-circuit die 104 must travel through the body 102 of thepackage and through the thickness of the walls of the package body toescape to the ambient. Depending on the thermal coefficient of the wallmaterial and the thickness of the wall, the thermal performance of thepackage is limited by the thermal resistance of the plasticpackage-molding material.

FIG. 4 shows a technique for improving the thermal performance of aplastic quad flat pack PQFP package assembly 150, which is similar tothe package assembly of FIG. 3 and where the same reference numerals areused for like elements. A PQFP package body 152 uses an internal heatspreader 154, which is a plate, or slug, formed of copper orcopper/tungsten material. The outside surface of the die-attach pad 106is bonded to the heat spreader 154, as indicated in the Figure. Use ofthe heat spreader 154 improves the heat transfer characteristics of thePQFP package assembly 150.

As in the case of the PGA package, the heat spreader 154 for the PQFPpackage is still in the series thermal path from the ambient to theintegrated-circuit chip 104. Cracks also develop in the body of thepackage 152 near the junction of the heat spreader 154 and the package152. As the metal of the heat spreader 154 expands and contracts withchanges in temperature, the stronger, more rigid metal heat spreader 154exerts stress on the package body 152 and on the integrated-circuit die104, causing the package material to crack. To reduce the amount ofstress on the package body, The heat spreader 154 is kept small,approximately the same size as the area of the integrated-circuit die104.

SUMMARY OF THE INVENTION

It is therefore an object of lite invention to provide a reliable,economical technique for providing direct access to the back of anintegrated-circuit chip packaged in a pin grid array PGA package or aplastic quad flat pack PQFP package so that various types ofdirect-cooling mechanisms can be applied directly to the back of thechip.

In accordance with this and other objects of the invention, theperipheral edge of a silicon chip is die-attached to a die-attach pad,formed as a ring within the package. The center of the ring, or pad, isopen to provide direct access to the exposed back side of a silicon chipfor a cooling media. The back side of the silicon chip can be metallizedfor sealing purposes. Various types of cooling media can be applieddirectly to the exposed back of the chip, including cooling-fluid flows,an external heatsink, and refrigeration units, using, for example, acooling fluid or a thermoelectric pump.

In one preferred embodiment of the invention, the integrated-circuitpackage body has a die-cavity formed therein. A die-attach pad has adie-attach surface portion formed adjacent the die-cavity in the packagebody. The die attach pad has an opening formed in the central portionthereof for exposing one side of the integrated-circuit so that anexternal cooling media can directly contact the one side of theintegrated-circuit die. The exposed side of the die is coated with afilm to provide a seal for the exposed side of the integrated-circuitdie.

In another preferred embodiment of the invention the package body has aring portion formed herein adjacent the peripheral edge of the diecavity and extending from the package body into the die cavity. Theperipheral edge of the integrated-circuit die is fixed to a mountingsurface of the ring portion of said package body to provide exposure ofthe one exposed side of said integrated-circuit die to accommodatedirect cooling of the exposed side of the integrated-circuit die. Themounting surface of the ring portion of the integrated-circuit packageextends outwardly beyond the peripheral edge of the integrated-circuitdie to accommodate a range of sizes of the integrated-circuit die.Various means for fluid cooling the exposed side of theintegrated-circuit die are provided, including an external heatsink,fluid coolants (such as air or water), and mechanical refrigerationunits (such as thermo-electric refrigeration units) in contact with theexposed side of the die.

The invention provides a method of directly cooling anintegrated-circuit die in a package. The method includes fixing theperipheral surface of one side of an integrated-circuit die to amounting surface formed on an inwardly extending portion of a packagebody, which has a cavity formed therein for accepting the integratedcircuit die. One side of the integrated-circuit die is exposed to theambient through an opening formed in the package body. In oneembodiment, the present invention includes the step of contacting theone exposed side of the integrated-circuit die with a cooling meanswhich can be a fluid cooling media, an external heatsink, or athermo-electric cooling device.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention:

FIG. 1 is a sectional view of a conventional, standard pin-grid-arrayPGA package configuration.

FIG. 2 is a sectional view of a conventional pin-grid-array PGA packageconfiguration with an internal heat spreader contained within thepackage.

FIG. 3 is a sectional view of a conventional, standard plastic quadflat-pack PQFP package configuration.

FIG. 4 is a sectional view of a conventional plastic quad flat-pack PQFPpackage configuration, also with an internal heat spreader containedwithin the package.

FIG. 5 is sectional view of a PGA packaging configuration according tothe invention, which has a die accessible from the back side of thepackage.

FIG. 6 is a sectional of an alternative embodiment of PGA packageconfiguration according to the invention, which can accommodate varioussizes of integrated-circuit dies, where the dies are directly accessiblefrom their back sides for cooling.

FIG. 7 is a sectional view of a PGA package configuration. according tothe invention which is cooled by a flow of cooling fluid, such as air,onto the exposed back side of an integrated-circuit die.

FIG. 8 is a sectional view of a PGA package configuration according tothe invention which is cooled, for example, by a thermoelectricrefrigeration unit.

FIG. 9 is a sectional view of a plastic quad flat pack PQFP packageconfiguration, which has an integrated-circuit die accessible from itsexposed back side.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims.

FIG. 5 shows an embodiment of a pin-grid-array PGA packagingconfiguration 200 according to the invention. In this configuration, anintegrated-circuit die 202 is directly accessible for cooling from theback surface, or side, of the body 204 of the package. A cavity 206 isformed into one face of the package body 204 for receiving theintegrated-circuit die 202, as indicated in the Figure. Extendingoutwardly from The body 204 is a shoulder, or ring portion 208. The ring208 has a die-mounting surface 210 on one side. To this one side isfixed the corresponding peripheral edge surface 212 of theintegrated-circuit die 202, using a suitable die-attach material 214,such as silver-filled epoxy, silver glass, or gold silicon eutectic. Thecavity 206 is extended through the body 204 of the package 200 toprovide an opening 216 adjacent the back surface, or back side, 218 ofthe integrated-circuit die 202, as indicated in the Figure. The opening216 provides for direct access to the back side 218 of theintegrated-circuit die 202. The back side of the die can be covered witha film of sealing material, such as gold or aluminum to seal the diefrom the ambient. As described hereinbelow, various means for directlycooling the back side 218 of the integrated-circuit die 202 areavailable.

The front surface, or side, 220 of the integrated-circuit die 202 hasvarious layers of materials formed and deposited thereupon to form thestructure of the various devices of the integrated circuit. A number ofbonding wires (typically shown as 222a, 222b, 222c and 220a, 220b, 220c)each have one end connected to various wire-bonding pads on the frontside 220 of the integrated-circuit die 202. The other ends of thevarious bonding wires 222a, 222b, 222c and 220a, 220b, 220c areconnected to respective bonding fingers of various internal, multilayerconductors (typically shown as 224a, 224b, 224c, and 226a, 226b, 226c).As shown in the Figure, the bonding fingers and associated conductorsare provided at various levels within the body of the package. Theinternal conductors 226a, 226b, 226c and 224a, 224b, 224c are connectedwith feed-through solder connections to respective PGA connector pins(not shown) which provide external connections to the integrated-circuitpackage. A lid 228 has its edges fixed to the front surface 230 of thepackage body 204 to seal the integrated-circuit die 202 within thedie-attach cavity 206.

FIG. 6 illustrates an embodiment of a PGA package configuration 250according to the invention. This configuration is designed toaccommodate integrated-circuit dies of various sizes, while stillpermitting the back side of a die to be accessed directly from its backside for cooling purposes. In this embodiment, a typicalintegrated-circuit die 252 is contained in a cavity 254 formed into thefront face of a package body 256, as indicated in the Figure. Adie-mounting ring 258 extends inwardly from the package body 256 intothe cavity 254. The ring 258 has a die-mounting surface 260 formed onits front side. The corresponding peripheral edge surface 262 of theintegrated-circuit die 252 is fixed to the die-mounting surface 260,using a suitable die-attach material 264. An opening 270 is providedthrough the ring 258 and is located adjacent the back surface, or side272 of the integrated-circuit die 252. The opening 270 provides fordirect access to the back side 272 of the integrated circuit die 252 fordirectly cooling the back side 272 of the integrated-circuit die 252.

The PGA package configuration 250 of FIG. 6 provides a single PGApackage body for mounting various sizes of integrated-circuit dies tothe mounting ring 258, while still permitting direct access to the backside of a die.

Various layers of materials are formed and deposited on the frontsurface, or side 274 to form the various structures of theintegrated-circuit die 252. To seal the exposed rear side 272 of theintegrated-circuit die 252, a film of a suitable material, such as goldor aluminum, is formed on the rear side 272 of the integrated-circuitdie 252.

Bonding wires (typically shown as 280, 282) each have one end connectedto a wire-bonding pad on the front side 274 of the integrated-circuitdie 252. The other ends of the bonding wires are connected to respectivebonding fingers of the internal conductors (typically shown as 284,286), which are connected with feed-through solder connections torespective pins 288, 290 to provide external connections to theintegrated-circuit package. A suitable lid (not shown) seals theintegrated-circuit die 252 within the die-attach cavity 254 in thepackage body 256.

FIG. 7 illustrates one of the various means for directly cooling theexposed back side 272 of the integrated-circuit die 252 contained in thePGA package configuration 250 illustrated in FIG. 6, according to theinvention. In this case, the arrows indicate a flow of cooling fluid,such as air or water, onto the exposed back side 272 of theintegrated-circuit die 252. The cooling fluid passes through the opening270 of FIG. 6 to directly contact the exposed back side 272 of theintegrated-circuit die 252.

FIG. 8 illustrates another of the various means for directly cooling theexposed back side 272 of the integrated-circuit die 252 contained in thePGA package configuration 250 illustrated in FIG. 6. A thermoelectricrefrigeration unit, or a conventional heatsink, generally represented byreference number 300, is fixed, for example, to the package body 256using a layer of heat-conductive, adhesive material 302. Thethermoelectric refrigeration unit 300 or conventional heatsink may beshaped as indicated in the Figure to have portions which conform to: theexposed back side 272 of the integrated-circuit die 252; the back sideof the ring 258; and the outer surface of the package body 256.

FIG. 9 illustrates a plastic quad flat pack package PQFP assembly 400having an integrated-circuit die 402 which can be accessed from itsexposed back side 404. The package assembly 400 includes a package body406 formed of molded plastic material and molded around theintegrated-circuit die 402. The integrated-circuit die 402 is fixed to adie-attach pad 408 with a suitable layer 409 of die-attach material, asindicated in the Figure.

The die-attach pad 408 has a central opening 410 formed therein. This isa modification from the conventional die-attach pad 106, such as shownin FIG. 3 of the drawings. The die-attach pad 408 is formed of a numberof different materials including: copper or copper alloy; ceramic;alumina nitride; silicon carbide; and glass epoxy. The package body 406is molded to have an opening 412 through the body 406 to expose the backside 404 of the integrated-circuit die 402 for direct cooling, asindicated in the Figure. Various means for directly cooling the backside 404 of the integrated-circuit die 402 are available, as described,for example, with reference to FIGS. 7 and 8 hereinabove.

Wire-bonding pads on the top surface of the integrated-circuit die 402are connected by bonding wires (typically shown as 414, 416) tocorresponding bonding finger portions of leads (typically shown as 418,420. The leads are embedded in the molded plastic body 406 and extendout the sides of the package body, as indicated in the Figure.

The various embodiments of the package configuration according to theinvention can be arranged either with the die-cavity located on the topof the package-that is, cavity-up, or with the die-cavity located on thebottom of the package-that is, cavity-down, as required.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and obviously manymodifications and variations are possible in light of the aboveteaching. The embodiments were chosen and described in order to bestexplain the principles of the invention and its practical application,to thereby enable others skilled in the art to best utilize theinvention and various embodiments with various modifications as aresuited to the particular use contemplated. It is intended that the scopeof the invention be defined by the Claims appended hereto and theirequivalents.

We claim:
 1. A method for directly cooling an integrated-circuit die,having a front and back surface, mounted in an integrally formed packageassembly comprising the steps of:fixing said back surface of anunpackaged integrated-circuit die to a mounting surface formed on aninwardly extending portion of a package body having a cavity formedtherein for accepting said unpackaged integrated-circuit die to form anintegral package assembly of said previously unpackaged integratedcircuit die and said package body; and exposing said back surface ofsaid fixed integrated-circuit die through a central opening formed insaid package body to provide direct cooling to said back surface of saidintegrated circuit die in said integral package assembly.
 2. The methodof claim 1 including the step of directly contacting said exposed backsurface of said integrated-circuit die with cooling means.
 3. The methodof claim 2 including the step of directly contacting said exposed backsurface of said integrated-circuit die with a fluid cooling media. 4.The method of claim 2 including the step of directly contacting saidexposed back surface of said integrated-circuit die with a heat sinkcoupled to said exposed back surface of said integrated-circuit die. 5.The method of claim 2 including the step of directly cooling saidexposed back surface of said integrated-circuit die with athermo-electric cooling mechanism.
 6. A method for directly cooling anintegrated-circuit die, having a front and back surface, mounted in anintegrally formed package assembly comprising the steps offfixing saidback surface of an unpackaged integrated-circuit die to a mountingsurface formed on an inwardly extending portion of a package body havinga cavity formed therein for accepting said unpackaged integrated-circuitdie to form an integral package assembly of said previously unpackagedintegrated circuit die and said package body; and exposing said backsurface of said integrated-circuit die through a central opening formedin said package body to provide direct cooling to said back surface ofsaid integrated circuit die in said integral package assembly: anddirectly contacting said exposed back surface of said integrated-circuitdie with cooling means.
 7. The method of claim 6 including the step ofdirectly contacting said exposed back surface of said integrated-circuitdie with a fluid cooling media.
 8. The method of claim 6 including thestep of directly contacting said exposed back surface of saidintegrated-circuit die with a heat sink coupled to said exposed backsurface of said integrated-circuit die.
 9. The method of claim 6including the step of directly cooling said exposed back surface of saidintegrated-circuit die with a thermo-electric cooling mechanism.
 10. Amethod for directly cooling an integrated-circuit circuit die, having afront and back surface, mounted in an integrally formed package assemblycomprising the steps of:fixing said back surface of an unpackagedintegrated-circuit die to a mounting surface formed on an inwardlyextending portion of a package body having a cavity formed therein foraccepting said unpackaged integrated-circuit die to form an integralpackage assembly of said previously unpackaged integrated circuit dieand said package body; and exposing said back surface of said fixedintegrated-circuit die through a central opening formed in said packagebody to provide direct cooling to said back surface of said integratedcircuit die in said integral package assembly; and directly contactingsaid exposed back surface of said integrated-circuit die with a heatsink coupled to said exposed back surface of said integrated-circuitdie.